After the cutscene there will be a quest icon marked on the map. Just travel there to update the quest. It is possible to avoid most of the undead and killing them is not needed to advance the mission, but there will be undead on top of the marked spots. After reaching the spot another marker will appear, and after that one a third marker will appear. At this third marker is Howl. Kill him and Rissa appears. Kill her then walk up to the dragon crystal in the cave behind her. Use the crystal to apply the poison and complete the mission.

Just remember, Mav: a good leader stops talking every now and then to listen.

Reeva: I love it when the warband goes out fighting. I hate the reason, but you know me: I like to find the upside.

I'll see the upside after we put Howl and that necromancer down for good.

Speaking to other NPCs:

Scholar Yahala Scribepaw: So, to review: this device will call out any large pockets of necromantic energy. That should lead us to the dragon crystal, which we will then destroy with our potion. Ready?

Ready. Let's go.

Crusader Gahn Towerbreaker: That was brutal. We lost some of our best soldiers out there.

This is war—but the Priory is working to win it with fewer casualties.

When approaching Korukhan the Blacksmith:

Whispers Agent: Multiple undead contacts sighted!

Whispers Agent: A horde of undead in the swamp!

Whispers Agent: Too many undead for me to handle.

Whispers Agent: Disgusting undead everywhere! And they smell worse than the most rotted rot I can imagine.

When approaching Howl the Brazen:

Scholar Yahala Scribepaw: (groan) I think we just located your fallen comrade. And the amulet he wore.

Upon reaching Howl the Brazen (cinematic):

<Character name>: Howl! Can you hear me? Do you understand? Give me a sign that you're still you, or I'll have to...look, just say something!

I helped Yahala and the Durmand Priory locate and destroy the dragon crystal. We also defeated the necromancer Rissa and the undead monstrosity that she made out of my dead warbandmate, Howl the Brazen. Yahala told me that my new Priory mentor is waiting in Lion's Arch, and I agreed to go there to begin my training as a Priory novice.